1. Field of the Invention
This invention relates generally to helical-scan type apparatus for recording and/or reproducing information signals, such as, a video signal and a PCM (pulse-code modulated) audio signal, in respective parts of slant tracks on a record tape by means of a rotary head, and more particularly is directed to an arrangement in such apparatus for also recording index signals in the slant tracks so as to indicate the position along the tape of a starting point of a recorded event or program.
2. Description of the Prior Art
As is shown on FIG. 1, a rotary head assembly of a typical 8 mm video tape recorder includes recording and reproducing rotary magnetic heads HA and HB. These rotary magnetic heads HA and HB have gaps with different azimuth angles and are mounted on a rotary drum 1 with an angular spacing of 180.degree. therebetween. Rotary heads HA and HB are rotated at a rotational speed equal to the standard frame frequency (30 Hz) in the direction indicated by 3 H and protrude slightly from the peripheral surface of the rotary drum 1. A magnetic tape 2 is wrapped around the peripheral surface of rotary drum 1 over an angular extent of 221.degree. and is suitably transported at a constant speed in the direction indicated by an arrow 3T during recording and normal reproducing operations.
Accordingly, tracks 4A and 4B, each having a length corresponding to an angular extent of 221.degree. on the surface of drum 1, are alternately formed on tape 2 by rotary heads HA and HB, respectively, as shown in FIG. 2. In a normal PCM mode of the 8 mm video tape recorder, a so-called overscan area AP of each track corresponding to an angular extent of 36.degree. measured from a point at which the rotary head HA or HB starts scanning the track 4A or 4B, has recorded therein an audio signal corresponding to one field period of the video signal and which has been pulse-code modulated and time compressed. Within the succeeding main area AV of each track having a length corresponding to an angular extent of 180.degree. there is recorded a color video signal of one field period. The remaining or concluding area of each track which corresponds to an angular extent of 5.degree. is assigned as a spare or trace ending area at which the head is disengaged from the track.
Further, the PCM audio signal can be recorded and reproduced by the 8 mm video tape recorder in a so-called "multi-PCM" or "audio-use" mode in which, as disclosed in detail in U.S. Pat. No 4,542,419, issued on Sept. 17, 1985 and having a common assignee herewith, the main recording area AV of each track is also used for recording PCM audio signals. In such audio-use or multi-PCM mode, the 8 mm video tape recorder becomes exclusively a PCM audio signal recording and/or reproducing apparatus in which PCM audio signals can be recorded in multiple channels. More specifically, as shown in FIG. 3, in the audio-use or multi-PCM mode, the area AV of each track corresponding to the angular range of 180.degree. is divided equally by five and the PCM audio signal is recorded in a selected one of these divided portions during scanning of each track. Therefore, the whole area of each track shown on FIG. 2, including the area AP and the area AV, is divided into six track areas AP1 to AP6, each corresponding to an angular range or extent of 36.degree., and first to sixth track channels are formed by the same numbered segments or track areas AP1 to AP6 of the respective skewed tracks 4A, 4B, 4A, 4B, etc., shown on FIG. 3, whereby a suitably time compressed PCM audio signal can be recorded in any desired channel. For example, the PCM audio signal may be recorded first in the segments or track areas AP1 of the successive tracks, 4A, 4B, from one end of the tape to the other. Thereafter, the PCM audio signal may be further recorded in segments or track areas AP2 of the successive tracks from one end of the tape to the other. Accordingly, PCM audio signals can be recorded in and reproduced from each of the six channels with the result that, in the audio-use or multi-PCM mode shown on FIG. 3, the tape 2 has an audio recording time or capacity six times as long as when a PCM audio signal is recorded only in the overscan area AP of each track, as on FIG. 2.
In each case of the multi-PCM mode, the PCM signal processing circuit used for each of the multiple channels may be the same signal processing circuit provided for processing the PCM audio signal recorded in the single channel constituted by the track areas AP in the prior art 8 mm video tape recorder.
The track format of the above mentioned 8 mm video tape recorder in its normal PCM mode will now be described more fully with reference to FIG. 4, in which contact of the rotary head with the tape 2, that is, the starting point of the track, begins at the left-hand side where there is provided a contact starting area having a length corresponding to a head rotation angle of 5.degree., and which consists of a tracking start area 21 of 2.94.degree. and an area 22 of 2.06.degree. corresponding to three horizontal periods (3 H) of the video signal and which is assigned as a preamble area for use as a clock run-in area synchronized with the succeeding PCM data. Following the preamble or clock run-in area 22, there is provided a PCM data recording area 23 having an angular extent of 26.32.degree. and in which a time compressed PCM audio signal is recorded. A postamble area 24 also having an angular extent of 2.06.degree. (3 H) follows the PCM data recording area 23 so as to be used as a back margin area to cope with the displacement of the recording position when the recording is carried out in the so-called after-recording mode. A next area 25 having an angular extent of 2.62.degree. is assigned as a guard band area for separating the video signal area 26 which follows from the PCM data area 23. The video recording area 26 has an angular range or extent of 180.degree. as earlier noted for receiving the recorded color video signal of one field period. Following the area 26, there is provided a head disengaging area 27 having the angular extent of 5.degree. and in which the rotary head is disengaged or separated freely from the magnetic tape.
The PCM data recorded in each recording area 23 is formed by interleaving audio data of one field period and index data associated with the audio data and encoding the same. The index data is formed by coding various identifying information, such as, the recording date, absolute address, program number and the so-called time code. Such index data is useful for locating a starting point of a recorded event or program for reproduction or editing.
However, if the index data is interleaved with the audio data and encoded in a mixed state, as described above, the index data cannot be extracted without de-interleaving the PCM data and then decoding the same. Thus, when the index data is used in a program search or editing operation, it takes considerable time to process the index data.
Further, in a search operation, the magnetic tape 2 moves at high speed, for example, 30 times the standard tape speed in the recording mode, so that the rotary head traces obliquely across the recording area 23. As a result, it is difficult to correctly reproduce the index data recorded therein with the audio data.
To solve the foregoing problems, it has been proposed, for example, as disclosed in the parent U.S. patent application Ser. No. 06/885,203, filed July 14, 1986, and having a common assignee herewith, to provide coded index data similar to the index data recorded with the PCM audio data in each recording area 23, and which is recorded in the postamble area 24 following the PCM signal recording area 23.
Recording of index data in the postamble area 24 will now be further explained with reference to FIGS. 5A-5F which illustrate a desired recording format with increasing detail. As shown in FIGS. 5A and 5B, the postamble area 24, having a length of three horizontal periods, that is, of 3 H, is further segmented into a first half-area 241 of 1.5 H and a second half-area 242 of 1.5 H. The half-area 241 is used to record a postamble signal for the PCM data and the half-area 242 is an index area used to record and/or reproduce the coded index data. As shown in FIG. 5C, the index area 242 is still further divided into an initial preamble signal area 242.sub.1 of 0.5 H and a subsequent index data area 242.sub.2 of 1 H in which coded index data CDIX is recorded.
The coded index data CDIX is formed of 7 blocks comprised of blocks BLK1 to BLK6 and an auxiliary block BAUX as shown in FIG. 5D. The auxiliary block BAUX is employed, for example, as an end mark. As shown in FIG. 5E, each of the blocks BLK1 to BLK6 comprises a block header of 3 bits, ID codes ID0 to ID4 each comprised of 8 bits, and a CRC code CRCC of 8 bits. The CRC code is what might be called a cyclic redundancy check code used to detect errors contained in the ID codes.
The ID codes ID0 to ID4 are exactly the same as the ID codes ID0 to ID4 recorded with the PCM audio data in the area 23 and indicate data, such as, a cut number, recording date and time, the absolute position on the magnetic tape 2 or the like.
Generally, the PCM data is recorded in the area 23 on the tape with the binary code data having a logic level "1" or "0" being modulated to signals of respective frequencies. In the 8 mm video tape recorder, for example, the data having the logic level "1" is modulated to, or represented by a recorded signal having a frequency of 5.8 MHz, while the data having the logic level "0" is modulated to, or represented by a recorded signal having a frequency 2.9 MHz.
Similarly, the index data CDIX is converted into a bi-phase signal which is recorded on the magnetic tape 2, and which has a frequency of 2.9 MHz for logic "0" and 5.8 MHz for logic "1".
If the index data CDIX is recorded in the index area 242 independent of the PCM data area 23 as mentioned hereinabove, it is possible to immediately obtain the index data by tracing the postamble area 24 in the reproducing mode. Even in the search mode with a tape speed 30 times as high as that in the recording mode, the index area 242 is short, that is it corresponds to only 1.5 H, so that the rotary head can positively reproduce the index data CDIX recorded therein. Further, since the index data CDIX is recorded in the index area 242 which is provided independently of the PCM data area 23 and separated from the latter by the postamble area 241, the index data can be independently and easily recorded in the index area 242 within the postamble area 24 of the recorded track in an after-recording mode, that is, a mode in which the index data is recorded after recording of the PCM data.
If the index signal is merely to represent a relative position on the tape, that is, if the index signal is merely indicative of the starting point of a tune, event, program or the like but does not absolutely locate such starting point, it is only necessary that the index signal be discriminated from the video signal and the PCM audio data which make up the information signal recorded on the tape. Thus, for example, if a postamble signal recorded in postamble area 24 is a single tone signal having a frequency of 5.8 MHz and thus corresponding to PCM data of all "1"s, an index signal which can be readily discriminated from this postamble signal and which has a very small probability of appearing as PCM audio data can be a single tone signal having a frequency of 2.9 MHz and which corresponds to data consisting of consecutive "0"s only. In such so-called "0" index system, the index signal generator ca be simplified and provided at low cost so that it can be used in a relatively inexpensive version of a recording and/or reproducing apparatus.
However, in the "0" index system, that is, where only relative position information is provided, the actual position along the tape can be determined from such relative position information only by counting the number of the index signals as the tape is searched from its head or leading end. However, if a tape cassette is ejected from a video tape recorder and then loaded therein again, and the starting point of a recorded event is searched beginning from a point midway between the ends of the tape, it is impossible to know the count of the first detected index signal measured from the head end of the tape.
Therefore, it is desirable to achieve compatibility between the "0" index system and the coded index system. To achieve such compatibility, it has been proposed, for example, as disclosed in the previously more completely identified U.S. patent application Ser. No. 06/885,203, to record both coded index data and "0" index data in successive halves, respectively, for example, in the areas 241 and 242, of the postamble area 24. Since each of the areas 241 and 242 has a length corresponding to 1.5 H, they are adequate for the recording therein of the coded index data and the "0" index data. However, by using the area 241 for recording of the coded index data, the postamble area recorded with "1"s is omitted between the recorded PCM audio data in area 23 and the coded, index data of similar format in area 241 with the result that the end of the PCM audio data in area 23 may not be readily discriminated. In order to permit retention of the postamble area 241 recorded with successive "1"s , consideration may be given to the use, as a "0" index area, of the preamble area 242.sub.1 (0.5 H) constituting the initial part of the index area 242 (FIG. 5C). In other words, the pre-amble area 242.sub.1, which is generally recorded with logic "1", is rewritten as logic "0". However, the preamble area 242.sub.1 of 0.5 H is represented as 210 bits at the most.
In respect to the foregoing, it should be noted that, in the "0" index system, consecutive data "0"s of more than a predetermined number have to be written so as to be surely discriminated from the information signal data. For example, a detecting circuit at the reproducing side may have to detect consecutive "0"s of more than 256 bits in order to surely indentify the starting point of the recorded event or program. In that case, if consecutive "0"s of no more than 210 bits are written in the preamble area 242.sub.1, they cannot be surely detected as the "0" index.